Direct monitoring of in vivo ER stress during the development of insulin resistance with ER stress-activated indicator transgenic mice

Metabolomic and transcriptomic analyses of Fmo5-/- mice reveal roles for flavin-containing monooxygenase 5 (FMO5) in NRF2-mediated oxidative stress response, unfolded protein response, lipid homeostasis, and carbohydrate and one-carbon metabolism

Flavin-containing monooxygenase 5 (FMO5) is a member of the FMO family of proteins, best known for their roles in the detoxification of foreign chemicals and, more recently, in endogenous metabolism. We have previously shown that Fmo5-/- mice display an age-related lean phenotype, with much reduced weight gain from 20 weeks of age. The phenotype is characterized by decreased fat deposition, lower plasma concentrations of glucose, insulin and cholesterol, higher glucose tolerance and insulin sensitivity, and resistance to diet-induced obesity. In the present study we report the use of metabolomic and transcriptomic analyses of livers of Fmo5-/- and wild-type mice to identify factors underlying the lean phenotype of Fmo5-/- mice and gain insights into the function of FMO5. Metabolomics was performed by the Metabolon platform, utilising ultrahigh performance liquid chromatography-tandem mass spectroscopy. Transcriptomics was performed by RNA-Seq and results analysed by DESeq2. Disruption of the Fmo5 gene has wide-ranging effects on the abundance of metabolites and expression of genes in the liver. Metabolites whose concentration differed between Fmo5-/- and wild-type mice include several saturated and monounsaturated fatty acids, complex lipids, amino acids, one-carbon intermediates and ADP-ribose. Among the genes most significantly and/or highly differentially expressed are Apoa4, Cd36, Fitm1, Hspa5, Hyou1, Ide, Me1 and Mme. The results reveal that FMO5 is involved in upregulating the NRF2-mediated oxidative stress response, the unfolded protein response and response to hypoxia and cellular stress, indicating a role for the enzyme in adaptation to oxidative and metabolic stress. FMO5 also plays a role in stimulating a wide range of metabolic pathways and processes, particularly ones involved in lipid homeostasis, the uptake and metabolism of glucose, the generation of cytosolic NADPH, and in one-carbon metabolism. The results predict that FMO5 acts by stimulating the NRF2, XBP1, PPARA and PPARG regulatory pathways, while inhibiting STAT1 and IRF7 pathways.

KIRA8 attenuates non-alcoholic steatohepatitis through inhibition of the IRE1α/XBP1 signalling pathway

Endoplasmic reticulum (ER) stress is enhanced in non-alcoholic steatohepatitis (NASH). Among three signalling pathways, the IRE1α/XBP1 signalling pathway is strongly implicated in the pathogenesis of NASH but its significance is still largely uncharacterised. In this report, we constructed a hepatocyte-specific XBP1-Luciferase knock-in mouse model that allows in vivo monitoring of the IRE1α/XBP1 activity in hepatocytes. Using this mouse model, we found that IRE1α/XBP1 was activated within hepatocytes during the pathogenesis of NASH. Significantly, a specific IRE1α kinase-inhibiting RNase attenuator, KIRA8, attenuated NASH in mice. In conclusion, our hepatocyte-specific XBP1 splicing reporter mouse represents a valid model for research and drug development of NASH, which showed that the IRE1α-induced XBP splicing is potentiated in hepatocytes during pathogenesis of NASH. Furthermore, we carried out the proof-of-concept study to demonstrate that the allosteric IRE1α RNase inhibitor serves as a promising therapeutic agent for the treatment of NASH.

Phytochemical Composition, Anti-Inflammatory and ER Stress-Reducing Potential of Sambucus ebulus L. Fruit Extract

Sambucus ebulus L. (SE) fruits are used for their immunostimulation, hematopoietic and antiviral potential. Recently, we focused on analyzing the mechanism underlying SE fruit aqueous extract's (FAE) immunomodulation and anti-inflammatory activities, with attention to its endoplasmic reticulum (ER) stress-reducing potential. J774A.1 macrophages were treated with SE FAE alone or in conditions of lipopolysaccharides (LPS) stimulation. Using GC-MS and LC-MS/MS, its phytochemical composition was analyzed. To measure transcription and protein levels, we used qPCR and Western blot, respectively. The prevailing phytochemicals in SE FAE were hydroxycinnamic acids, proanthocyanidins and anthocyanins. The content of some amino acids, organic acids, alcohols, fatty acids and esters were newly reported. Extracts exerted an immunostimulation potential by stimulating IL-6, TNFα, Ccl2, COX2 and iNOS transcription, without inducing ER stress. SE FAE suppressed the LPS-induced transcription of inflammation related genes (IL-1β, IL-6, TNFα, Ccl2, Icam-1, Fabp4, COX2, iNOS, Noxo1, IL-1ra, Sirt-1) and reduced the protein levels of iNOS, peIF2α, ATF6α and CHOP. The effects were comparable to that of salicylic acid. SE suppresses LPS-stimulated inflammatory markers on the transcription and translation levels. Targeting ER stress is possibly another mechanism underlying its anti-inflammatory potential. These findings reveal the potential of SE fruits as a beneficial therapeutic of inflammation and ER stress-related pathological conditions.

[Endoplasmic reticulum stress response and pathogenesis of non-alcoholic steatohepatitis]

The incidence of chronic liver disease is constantly increasing, owing to the obesity epidemic. Non-alcoholic fatty liver disease (NAFLD) is currently affecting 20-30% of the general population and 75-100% of obese individuals. NAFLD ranges from simple steatosis to damaging non-alcoholic steatohepatitis (NASH), potentially developing into hepatocellular carcinoma. No efficient pharmacological treatment is yet available. During obesity, the hepatic ER stress response can arise from extracellular stress (lipids, glucose, cytokines) and from intracellular stress including lipid buildup in the hepatocyte (steatosis), a hallmark of NAFLD. The chronic activation of the hepatic ER stress response may be a crucial event in the steatosis-NASH transition, triggering cell death, inflammation and accelerating metabolic disorders. We discuss these aspects and we propose that targeting the ER stress response could be effective in treating NAFLD.

Endoplasmic reticulum stress in the livers of BDNF heterozygous knockout mice

Context: Involvement of endoplasmic reticulum (ER) stress and brain-derived neurotrophic factor (BDNF) in hepatic lipid metabolism has been reported previously. Objective: The effects of chronic BDNF deficiency on ER stress response in the livers were examined in this study. Methods: BDNF^(+/-) mice, characterised by BDNF deficiency, and their wild-type (WT) littermates were used. The ER stress was induced by tunicamycin (Tm) (0.5 mg/kg, intraperitoneal). Animals were divided into four groups; WT, WT + Tm, BDNF^(+/-), and BDNF^(+/-)+Tm. Results: At the basal conditions, BDNF deficiency did not affect hepatic cell death or lipid accumulation. However, during ER stress, BDNF^(+/-)+Tm group showed increased apoptosis, GADD153 immunostaining, sterol regulatory element-binding protein-1c (SREBP-1c) level, and steatosis compared to the WT + Tm group. Conclusion: Endogenous BDNF might be protective against apoptosis through GADD153 suppression and steatosis via SREBP-1c suppression during ER stress. This effect of BDNF might be clinically important for type 2 diabetes and obesity, which are related with both ER stress and BDNF deficiency.

Flaxseed Oil Attenuates Hepatic Steatosis and Insulin Resistance in Mice by Rescuing the Adaption to ER Stress

Increasing evidence has demonstrated the benefits of α-linolenic acid-rich flaxseed oil (ALA-FO) against lipid metabolism abnormality in both rodent models and humans. However, the metabolic response of FO to insulin resistance and type 2 diabetes is still inconsistent. This study aimed to explore the effect of FO on chronic high fat diet (HFD)-induced hepatic steatosis, insulin resistance, and inflammation, mainly focusing on hepatic n-3 fatty acid remodeling and endoplasmic reticulum (ER) unfolded protein response. The results showed that lard-based HFD feeding for 16 weeks (60% fat-derived calories) induced whole-body insulin resistance, lipid profile abnormality, and inflammation in mice, which was alleviated by FO in a dose-dependent manner. Moreover, FO effectively improved hepatic steatosis and insulin resistance in mice by modulating the specific location of ALA and its long-chain n-3 fatty acids across hepatic lipid fractions and enhancing insulin-stimulated phosphorylation of hepatic insulin receptor subtract-1 (IRS-1) tyrosine 632 and protein kinase B (AKT) ( p < 0.05). Importantly, the differential depositions of ALA and its long-chain n-3 fatty acids in plasma and ER membranes were observed, concomitant with the rescued ER unfolded protein response and Jun N-terminal kinase (JNK) signaling in mice liver.

Endoplasmic reticulum stress signalling and the pathogenesis of non-alcoholic fatty liver disease

The global epidemic of obesity has been accompanied by a rising burden of non-alcoholic fatty liver disease (NAFLD), with manifestations ranging from simple steatosis to non-alcoholic steatohepatitis, potentially developing into hepatocellular carcinoma. Although much attention has focused on NAFLD, its pathogenesis remains largely obscure. The hallmark of NAFLD is the hepatic accumulation of lipids, which subsequently leads to cellular stress and hepatic injury, eventually resulting in chronic liver disease. Abnormal lipid accumulation often coincides with insulin resistance in steatotic livers and is associated with perturbed endoplasmic reticulum (ER) proteostasis in hepatocytes. In response to chronic ER stress, an adaptive signalling pathway known as the unfolded protein response is triggered to restore ER proteostasis. However, the unfolded protein response can cause inflammation, inflammasome activation and, in the case of non-resolvable ER stress, the death of hepatocytes. Experimental data suggest that the unfolded protein response influences hepatic tumour development, aggressiveness and response to treatment, offering novel therapeutic avenues. Herein, we provide an overview of the evidence linking ER stress to NAFLD and discuss possible points of intervention.

Liver function and dysfunction - a unique window into the physiological reach of ER stress and the unfolded protein response

The unfolded protein response (UPR) improves endoplasmic reticulum (ER) protein folding in order to alleviate stress. Yet it is becoming increasingly clear that the UPR regulates processes well beyond those directly involved in protein folding, in some cases by mechanisms that fall outside the realm of canonical UPR signaling. These pathways are highly specific from one cell type to another, implying that ER stress signaling affects each tissue in a unique way. Perhaps nowhere is this more evident than in the liver, which-beyond being a highly secretory tissue-is a key regulator of peripheral metabolism and a uniquely proliferative organ upon damage. The liver provides a powerful model system for exploring how and why the UPR extends its reach into physiological processes that occur outside the ER, and how ER stress contributes to the many systemic diseases that involve liver dysfunction. This review will highlight the ways in which the study of ER stress in the liver has expanded the view of the UPR to a response that is a key guardian of cellular homeostasis outside of just the narrow realm of ER protein folding.

Melatonin Modulates Neuronal Cell Death Induced by Endoplasmic Reticulum Stress under Insulin Resistance Condition

Insulin resistance (IR) is an important stress factor in the central nervous system, thereby aggravating neuropathogenesis and triggering cognitive decline. Melatonin, which is an antioxidant phytochemical and synthesized by the pineal gland, has multiple functions in cellular responses such as apoptosis and survival against stress. This study investigated whether melatonin modulates the signaling of neuronal cell death induced by endoplasmic reticulum (ER) stress under IR condition using SH-SY5Y neuroblastoma cells. Apoptosis cell death signaling markers (cleaved Poly [ADP-ribose] polymerase 1 (PARP), p53, and Bax) and ER stress markers (phosphorylated eIF2α (p-eIF2α), ATF4, CHOP, p-IRE1, and spliced XBP1 (sXBP1)) were measured using reverse transcription-PCR, quantitative PCR, and western blottings. Immunofluorescence staining was also performed for p-ASK1 and p-IRE1. The mRNA or protein expressions of cell death signaling markers and ER stress markers were increased under IR condition, but significantly attenuated by melatonin treatment. Insulin-induced activation of ASK1 (p-ASK1) was also dose dependently attenuated by melatonin treatment. The regulatory effect of melatonin on neuronal cells under IR condition was associated with ASK1 signaling. In conclusion, the result suggested that melatonin may alleviate ER stress under IR condition, thereby regulating neuronal cell death signaling.

Targeting endoplasmic reticulum stress in liver disease

INTRODUCTION

The accumulation of unfolded protein in the endoplasmic reticulum (ER) initiates an unfolded protein response (UPR) via three signal transduction cascades, which involve protein kinase RNA-like ER kinase (PERK), inositol requiring enzyme-1α (IRE1α) and activating transcription factor-6α (ATF6α). An ER stress response is observed in nearly all physiologies related to acute and chronic liver disease and therapeutic targeting of the mechanisms implicated in UPR signaling have attracted considerable attention.

AREAS COVERED

This review focuses on the correlation between ER stress and liver disease and the possible targets which may drive the potential for novel therapeutic intervention. Expert Commentary: We describe pathways which are involved in UPR signaling and their potential correlation with various liver diseases and underlying mechanisms which may present opportunities for novel therapeutic strategies are discussed.

Insulin resistance contributes to multidrug resistance in HepG2 cells via activation of the PERK signaling pathway and upregulation of Bcl-2 and P-gp

Liver tumorigenesis frequently causes insulin resistance which may be used as an independent risk factor for evaluation of survival and post-surgery relapse of liver cancer patients. In the present study, HepG2/IR, an insulin resistant HepG2 cell line, was established by exposing HepG2 cells to 0.5 µmol/l of insulin for 72 h, and comparison of HepG2/IR with the parental HepG2 cells indicated that the HepG2/IR cells showed significantly enhanced resistance to the most frequently used chemotherapeutics for solid tumors, such as cisplatin, 5-fluorouracil, vincristine and mitomycin. Flow cytometric analysis of cisplatin-treated HepG2/IR cells showed a significantly decreased hypodiploid peak and a significantly downregulated expression level of pro-apoptotic protein caspase-3 compared with the parental HepG2 cells. Our data further showed swollen endoplasmic reticulum (ER) in the cisplatin-treated HepG2/IR cells with significantly increased levels of glucose-regulated protein 78 (GRP78), phosphorylated protein kinase R-like ER kinase (p-PERK) and P-glycoprotein (P-gp). There was also an upregulated expression of anti-apoptotic protein B-cell lymphoma 2 (Bcl-2) whereas no significant change was observed for CCAAT-enhancer-binding protein homologous protein (CHOP), which is known to be induced by ER stress and to mediate apoptosis. Our results demonstrated that insulin resistance in HepG2 cells promoted a protective unfolded protein response and upregulated the expression of ER chaperone protein GRP78, which resulted in the phosphorylation of PERK kinase to activate the PERK-mediated ER stress signal transduction pathway and the upregulation of Bcl-2 and P-gp, leading to the inhibition of the caspase-3-dependent apoptosis pathway and to the survival of liver tumor cells.

Role of NKG2D in obesity-induced adipose tissue inflammation and insulin resistance

The early events that initiate inflammation in the adipose tissue during obesity are not well defined. It is unclear whether the recruitment of CD8 T cells to the adipose tissue during onset of obesity occurs through antigen-dependent or -independent processes. We have previously shown that interaction between NKG2D (natural-killer group 2, member D) and its ligand Rae-1ε is sufficient to recruit cytotoxic T lymphocytes to the pancreas and induce insulitis. Here, we tested whether NKG2D–NKG2D ligand interaction is also involved in obesity-induced adipose tissue inflammation and insulin resistance. We observed a significant induction of NKG2D ligand expression in the adipose tissue of obese mice, especially during the early stages of obesity. However, mice lacking NKG2D developed similar levels of insulin resistance and adipose tissue inflammation compared to control mice when placed on a high-fat diet. Moreover, overexpression of Rae-1ε in the adipose tissue did not increase immune cell infiltration to the adipose tissue either in the setting of a normal or high-fat diet. These results indicate that, unlike in the pancreas, NKG2D–NKG2D ligand interaction does not play a critical role in obesity-induced inflammation in the adipose tissue.

Rosiglitazone attenuates casein-induced hepatic endoplasmic reticulum stress in Sprague-Dawley rats: a novel model of endoplasmic reticulum stress

The proteins found in cow milk have been reported to cause systemic inflammation. Endoplasmic reticulum (ER) stress is known to be involved in the development of several metabolic disorders including insulin resistance and non-alcoholic fatty liver disease. However, the effect of thiazolidinediones (TZDs) on ER stress is still controversial. This is why we want to investigate in this study whether casein, which is the major protein in cow's milk, induces ER stress in the liver and whether rosiglitazone can attenuate these changes. Nine-week-old male Sprague-Dawley (SD) rats were separated into three groups: (1) vehicle treated; (2) daily subcutaneous injections of 1 mL 10% casein; (3) daily subcutaneous injection of 1 mL 10% casein and rosiglitazone 4 mg/[kg d]. After 6 weeks, body weight, food intake, glucose and lipid parameters, and serum AST/ALT levels were measured after an overnight fast. Real time RT-PCR and immunohistochemical staining for various ER stress markers were performed, and a TUNEL analysis was also performed. After 6 weeks, casein injection induced weight reduction, systemic inflammation, and hepatic dysfunction in SD rats. Casein injection increased both the gene and protein expression of ER stress markers in the liver and also caused hepatocyte apoptosis. Rosiglitazone treatment attenuated casein-induced systemic inflammation, ER stress, deteriorated liver function, and increased apoptosis. In conclusion, our results may provide further insight into the effects of casein on chronic inflammatory diseases, and to have a better understanding of the mechanism of the anti-inflammatory properties of rosiglitazone regardless of its hypoglycemic effect.

Downregulation of chemerin and alleviation of endoplasmic reticulum stress by metformin in adipose tissue of rats

AIMS

To investigate whether metformin regulates chemerin expression in vivo by alleviating ER stress.

METHODS

Male Sprague-Dawley rats were fed a high-fat or normal diet for 10 weeks to induce insulin resistance. During the following 6 weeks, the rats were divided into four groups: normal diet without treatment (NC), normal diet with metformin treatment (NM), high-fat diet without metformin (HF), and high-fat diet with metformin (HM). Body weight, fasting glucose, basal insulin level, insulin sensitivity, chemerin expression in serum and adipose tissue, ER stress marker and its pathway were measured.

RESULTS

After 6 weeks treatment, metformin reduced the body weight gain and enhanced insulin sensitivity of high-fat fed rats. The basal insulin level in the HM group was lower than in the HF group. Metformin reduced chemerin expression in the HM group compared with HF. Metformin reduced the GRP78 mRNA expression in HM rats. Activation of IRE1 alpha was lower in the HM group than the HF group.

CONCLUSIONS

Metformin treatment decreased the chemerin expression and alleviated the ER stress in the visceral adipose tissue of high-fat diet-induced insulin-resistant rats. These data may also provide a further rationale for exploring the use of metformin in the treatment of insulin resistance.

Redox control of the survival of healthy and diseased cells

Abstract Cellular redox homeostasis is the first line of defense against diverse stimuli and is crucial for various biological processes. Reactive oxygen species (ROS), byproducts of numerous cellular events, may serve in turn as signaling molecules to regulate cellular processes such as proliferation, differentiation, and apoptosis. However, when overproduced ROS fail to be scavenged by the antioxidant system, they may damage cellular components, giving rise to senescent, degenerative, or fatal lesions in cells. Accordingly, this review not only covers general mechanisms of ROS production under different conditions, but also focuses on various types of ROS-involved diseases, including atherosclerosis, ischemia/reperfusion injury, diabetes mellitus, neurodegenerative diseases, and cancer. In addition, potentially therapeutic agents and approaches are reviewed in a relatively comprehensive manner. However, due to the complexity of ROS and their cellular impacts, we believe that the goal to design more effective approaches or agents may require a better understanding of mechanisms of ROS production, particularly their multifaceted impacts in disease at biochemical, molecular, genetic, and epigenetic levels. Thus, it requires additional tools of omics in systems biology to achieve such a goal. Antioxid. Redox Signal. 15, 2867-2908.

Stressed out about obesity: IRE1α-XBP1 in metabolic disorders

The global obesity epidemic is associated with a series of health-threatening diseases including type 2 diabetes. Accumulating evidence suggest that the physiology and homeostasis of the endoplasmic reticulum (ER) is intimately involved in the underlying mechanisms linking obesity and diabetes. Specifically, recent studies indicate a crucial role for the inositol-requiring enzyme 1α (IRE1α)/X-box binding protein 1 (XBP1) pathway, the most conserved branch of the unfolded protein response (UPR), in glucose and lipid metabolism as well as in insulin function. Focusing on the IRE1α-XBP1 pathway, we review recent advances in our understanding of the role of UPR in obesity and obesity-associated metabolic disorders.

The contribution of endoplasmic reticulum stress to liver diseases

The unfolded protein response (UPR) is an evolutionarily conserved cell signaling pathway that is activated to regulate protein synthesis and restore homeostatic equilibrium when the cell is stressed from increased client protein load or the accumulation of unfolded or malfolded proteins. Once activated, this signaling pathway can either result in the recovery of homeostasis or can activate a cascade of events that ultimately result in cell death. The UPR/endoplasmic reticulum (ER) stress response spectrum and its interplay with other cellular organelles play an important role in the pathogenesis of disease in secretory cells rich in ER, such as hepatocytes. Over the past 2 decades, the contribution of ER stress to various forms of liver diseases has been examined. Robust support for a contributing, as opposed to a secondary role, for ER stress response is seen in the nonalcoholic steatohepatitis, alcoholic liver disease, ischemia/reperfusion injury, and cholestatic models of liver disease. The exact direction of the cause and effect relationship between modes of cell injury and ER stress remains elusive. It is apparent that a complex interplay exists between ER stress response, conditions that promote it, and those that result from it. A vicious cycle in which ER stress promotes inflammation, cell injury, and steatosis and in which steatogenesis, inflammation, and cell injury aggravate ER stress seems to be at play. It is perhaps the nature of such a vicious cycle that is the key pathophysiologic concept. Therapeutic approaches aimed at interrupting the cycle may dampen the stress response and the ensuing injury.

Deletion of Nck1 attenuates hepatic ER stress signaling and improves glucose tolerance and insulin signaling in liver of obese mice

Obesity has been shown to create stress in the endoplasmic reticulum (ER), and that initiates the activation of the unfolded protein response (UPR). This has been reported to cause insulin resistance in selective tissues through activation of the inositol-requiring enzyme 1α (IRE1α)-c-Jun NH(2)-terminal kinase (JNK) pathway, which results in the phosphorylation of the insulin receptor substrate-1 (IRS-1) at an inhibitory site and blocks insulin receptor signaling. In this study, we report that the Src homology domain-containing adaptor protein Nck1, previously shown to modulate the UPR, is of functional importance in obesity-induced ER stress signaling and inhibition of insulin actions. We have examined obese Nck1(-/-) and Nck1(+/+) mice for glucose tolerance, insulin sensitivity, and signaling as well as for ER stress markers and IRS-1 phosphorylation at Ser(307). Our findings show that obese Nck1-deficient mice display improved glucose disposal accompanied by enhanced insulin signaling in liver. This correlates with attenuated IRE1α and JNK activation and IRS-1 phosphorylation at Ser(307) compared with obese wild-type mice. Consistent with our in vivo data, we report that downregulation of Nck1 using siRNA in HepG2 cells results in decreased thapsigargin-induced IRE1α activation and signaling and IRS-1 phosphorylation at Ser(307), whereas it markedly enhances insulin signaling. Overall, in liver and in cultured cells, we show that depletion of Nck1 attenuates the UPR signal and its inhibitory action on insulin signaling. Taken all together, our findings implicate Nck1 in regulating the UPR, which secondary to obesity impairs glucose homeostasis and insulin actions.

The unfolded protein response is activated in skeletal muscle by high-fat feeding: potential role in the downregulation of protein synthesis

High-fat diets are known to decrease muscle protein synthesis, the adaptation to overload, and insulin sensitivity. Conditions that disrupt endoplasmic reticulum (ER) homeostasis lead to the activation of the unfolded protein response (UPR) that is associated with decreases in protein synthesis, chronic inflammation, and insulin resistance. The purpose of the present study was to establish whether ER stress is induced by a high-fat diet in skeletal muscle and whether ER stress can decrease mTORC1 activity and protein synthesis in muscle cells. Two independent protocols of high-fat feeding activated the UPR in mice. In the first study, mice consuming a high-fat diet containing 70% fat and <1% carbohydrates for 6 wk showed higher markers of the UPR (BiP, IRE1α, and MBTPS2) in the soleus and in the tibialis anterior muscles and ATF4 in the tibialis anterior (P < 0.05). In the second study, a 20-wk high-fat diet containing 46% fat and 36% carbohydrates also increased BiP, IRE1α, and phospho-PERK protein and the expression of ATF4, CHOP, and both the spliced and unspliced forms of XBP1 in the plantar flexors (P < 0.05). In C(2)C(12) muscle cells, tunicamycin, thapsigargin, and palmitic acid all increased UPR markers and decreased phosphorylation of S6K1 (P < 0.05). Collectively, these data show that a high-fat diet activates the UPR in mouse skeletal muscle in vivo. In addition, in vitro studies indicate that palmitic acid, and other well-known ER stress inducers, triggered the UPR in myogenic cells and led to a decrease in protein synthesis and mTORC1 activity.

Taurine reduces ER stress in C. elegans

Background

ER stress is a strong indicator of whether or not a cell is undergoing physiological stress. C. elegans is a practical system of characterizing the effect of ER stress at the in vivo or organismal level.

Methods

This study characterized taurine’s anti-ER stress potential employing western blotting on ER stress markers and assays of motility, lifespan comparison, and fecundity measurement.

Results

When treated with tunicamycin, C. elegans showed the typical ER stress symptoms. It showed a higher expression of hsp-70 and skn-1 than the non-treated control. Survivorship significantly decreased under tunicamycin treatment, and the offspring number also decreased. During the synchronized culture under ER stress conditions, the C. elegans showed early signs of aging especially between L3 and L4 within their life span, along with lowered motility. The worms, however, showed a positive response to the taurine treatment under ER stress conditions.

Conclusions

When C. elegans were treated with taurine before or after the tunicamycin treatment, they showed a less severe level of ER stress, including an enhanced survivorship, increased motility, and augmented fecundity. Taken together, these results strongly indicate that taurine works positively to cope with ER stress from the organismal perspective.

The endoplasmic reticulum stress-C/EBP homologous protein pathway-mediated apoptosis in macrophages contributes to the instability of atherosclerotic plaques

OBJECTIVE

To elucidate whether and how the endoplasmic reticulum (ER) stress-C/EBP homologous protein (CHOP) pathway in macrophages is involved in the rupture of atherosclerotic plaques.

METHODS AND RESULTS

Increases in macrophage-derived foam cell death in coronary atherosclerotic plaques cause the plaque to become vulnerable, thus resulting in acute coronary syndrome. The ER stress-CHOP/growth arrest and DNA damage-inducible gene-153 (GADD153) pathway is induced in the macrophage-derived cells in atherosclerotic lesions and is involved in plaque formation. However, the role of CHOP in the final stage of atherosclerosis has not been fully elucidated. Many CHOP-expressing macrophages showed apoptosis in advanced ruptured atherosclerotic lesions in wild-type mice, whereas few apoptotic cells were observed in Chop(-/-) mice. The rupture of atherosclerotic plaques was significantly reduced in high cholesterol-fed Chop(-/-)/Apoe(-/-) mice compared with Chop(+/+)/Apoe(-/-) mice. Furthermore, using mice that underwent bone marrow transplantation, we showed that expression of CHOP in macrophages significantly contributes to the formation of ruptures. By using primary cultured macrophages, we further showed that unesterified free cholesterol derived from incorporated denatured low-density lipoprotein was accumulated in the ER and induced ER stress-mediated apoptosis in a CHOP-Bcl2-associated X protein (Bax) pathway-dependent manner.

CONCLUSIONS

The ER stress-CHOP-Bax-mediated apoptosis in macrophages contributes to the instability of atherosclerotic plaques.

Macrophages, inflammation, and insulin resistance

Obesity induces an insulin-resistant state in adipose tissue, liver, and muscle and is a strong risk factor for the development of type 2 diabetes mellitus. Insulin resistance in the setting of obesity results from a combination of altered functions of insulin target cells and the accumulation of macrophages that secrete proinflammatory mediators. At the molecular level, insulin resistance is promoted by a transition in macrophage polarization from an alternative M2 activation state maintained by STAT6 and PPARs to a classical M1 activation state driven by NF-kappaB, AP1, and other signal-dependent transcription factors that play crucial roles in innate immunity. Strategies focused on inhibiting the inflammation/insulin resistance axis that otherwise preserve essential innate immune functions may hold promise for therapeutic intervention.

A point mutation in Sec61alpha1 leads to diabetes and hepatosteatosis in mice

OBJECTIVE

Type 2 diabetes is caused by both environmental and genetic factors. To better understand the genetic factors we used forward genetics to discover genes that have not previously been implicated in the development of hyperglycemia or diabetes.

RESEARCH DESIGN AND METHODS

Offspring of ethylnitrosurea-mutagenized C57BL/6 mice were bred to homozygosity, maintained on high-fat diet, and screened for hyperglycemia. The phenotype in one diabetic family of mice was mapped among hybrid F2s with single nucleotide polymorphic markers, followed by candidate gene sequencing to identify the gene harboring the causative mutation. Subsequent analysis was done on wild-type, heterozygous, and homozygous mutant mice on a pure C57BL/6 background.

RESULTS

Diabetes mapped to a point mutation in the Sec61a1 gene that encodes a His to Tyr substitution at amino acid 344 (Y344H). Metabolic profiling, histological examination, and electron microscopy revealed that hyperglycemia was a result of insulin insufficiency due to beta-cell apoptosis brought on by endoplasmic reticulum (ER) stress. Transgenic beta-cell-specific expression of Sec61a1 in mutant mice rescued diabetes, beta-cell apoptosis, and ER stress. In vitro experiments showed that Sec61alpha1 plays a critical role in the beta-cell response to glucose.

CONCLUSIONS

Here we phenotypically characterize diabetes in mice with a novel point mutation in a basic component of the cell's ER protein translocation machinery, Sec61alpha1. Translocation by the mutant protein does not appear to be affected. Rather, ER homeostasis is perturbed leading to beta-cell death and diabetes.

Endoplasmic reticulum stress-induced apoptosis in the development of diabetes: is there a role for adipose tissue and liver?

Diabetes mellitus (DM) is a multifactorial chronic metabolic disease characterized by hyperglycaemia. Several different mechanisms have been implicated in the development of the disease, including endoplasmic reticulum (ER) stress. ER stress is increasingly acknowledged as an important mechanism in the development of DM, not only for β-cell loss but also for insulin resistance. Accumulating evidence suggests that ER stress-induced apoptosis may be an important mode of β-cell loss and therefore important in the development of diabetes. Recent data also suggest a role of ER stress-induced apoptosis in liver and adipose tissue in relation to diabetes, but more extensive studies on human adipocyte and hepatocyte (patho)physiology and ER stress are needed to identify the exact interactions between environmental signals, ER stress and apoptosis in these organs.

Emerging roles for XBP1, a sUPeR transcription factor

X-box binding protein 1 (XBP1) is a unique basic region leucine zipper (bZIP) transcription factor whose active form is generated by a nonconventional splicing reaction upon disruption of homeostasis in the endoplasmic reticulum (ER) and activation of the unfolded protein response (UPR). XBP1, first identified as a key regulator of major histocompatibility complex (MHC) class II gene expression in B cells, represents the most conserved signaling component of UPR and is critical for cell fate determination in response to ER stress. Here we review recent advances in our understanding of this multifaceted transcription factor in health and diseases.

Hepatic Bax inhibitor-1 inhibits IRE1alpha and protects from obesity-associated insulin resistance and glucose intolerance

The unfolded protein response (UPR) or endoplasmic reticulum (ER) stress response is a physiological process enabling cells to cope with altered protein synthesis demands. However, under conditions of obesity, prolonged activation of the UPR has been shown to have deteriorating effects on different metabolic pathways. Here we identify Bax inhibitor-1 (BI-1), an evolutionary conserved ER-membrane protein, as a novel modulator of the obesity-associated alteration of the UPR. BI-1 partially inhibits the UPR by interacting with IRE1alpha and inhibiting IRE1alpha endonuclease activity as seen on the splicing of the transcription factor Xbp-1. Because we observed a down-regulation of BI-1 expression in liver and muscle of genetically obese ob/ob and db/db mice as well as in mice with diet-induced obesity in vivo, we investigated the effect of restoring BI-1 expression on metabolic processes in these mice. Importantly, BI-1 overexpression by adenoviral gene transfer dramatically improved glucose metabolism in both standard diet-fed mice as well as in mice with diet-induced obesity and, critically, reversed hyperglycemia in db/db mice. This improvement in whole body glucose metabolism and insulin sensitivity was due to dramatically reduced gluconeogenesis as shown by reduction of glucose-6-phosphatase and phosphoenolpyruvate carboxykinase expression. Taken together, these results identify BI-1 as a critical regulator of ER stress responses in the development of obesity-associated insulin resistance and provide proof of concept evidence that gene transfer-mediated elevations in hepatic BI-1 may represent a promising approach for the treatment of type 2 diabetes.

Diabetes- and angiotensin II-induced cardiac endoplasmic reticulum stress and cell death: metallothionein protection

We have shown cardiac protection by metallothionein (MT) in the development of diabetic cardiomyopathy (DCM) via suppression of cardiac cell death in cardiac-specific MT-overexpressing transgenic (MT-TG) mice. The present study was undertaken to define whether diabetes can induce cardiac endoplasmic reticulum (ER) stress and whether MT can prevent cardiac cell death via attenuating ER stress. Diabetes was induced by streptozotocin in both MT-TG and wild-type (WT) mice. Two weeks, and 2 and 5 months after diabetes onset, cardiac ER stress was detected by expression of ER chaperones, and apoptosis was detected by CCAAT/enhancer-binding protein (C/EBP) homologous protein (CHOP) and cleaved caspase-3 and caspase-12. Cardiac apoptosis in the WT diabetic mice, but not in MT-TG diabetic mice, was significantly increased 2 weeks after diabetes onset. In parallel with apoptotic effect, significant up-regulation of the ER chaperones, including glucose-regulated protein (GRP)78 and GRP94, cleaved ATF6 and phosporylated eIF2alpha, in the hearts of WT, but not MT-TG diabetic mice. Infusion of angiotensin II (Ang II) also significantly induced ER stress and apoptosis in the hearts of WT, but not in MT-TG mice. Direct administration of chemical ER stress activator tunicamycin significantly increased cardiac cell death only in WT mice. Pre-treatment with antioxidants completely prevented Ang II-induced ER stress and apoptosis in the cultured cardiac cells. These results suggest that ER stress exists in the diabetic heart, which may cause the cardiac cell death. MT prevents both diabetes- and Ang II-induced cardiac ER stress and associated cell death most likely via its antioxidant action, which may be responsible for MT's prevention of DCM.

Maternal high-fat diet triggers lipotoxicity in the fetal livers of nonhuman primates

Maternal obesity is thought to increase the offspring's risk of juvenile obesity and metabolic diseases; however, the mechanism(s) whereby excess maternal nutrition affects fetal development remain poorly understood. Here, we investigated in nonhuman primates the effect of chronic high-fat diet (HFD) on the development of fetal metabolic systems. We found that fetal offspring from both lean and obese mothers chronically consuming a HFD had a 3-fold increase in liver triglycerides (TGs). In addition, fetal offspring from HFD-fed mothers (O-HFD) showed increased evidence of hepatic oxidative stress early in the third trimester, consistent with the development of nonalcoholic fatty liver disease (NAFLD). O-HFD animals also exhibited elevated hepatic expression of gluconeogenic enzymes and transcription factors. Furthermore, fetal glycerol levels were 2-fold higher in O-HFD animals than in control fetal offspring and correlated with maternal levels. The increased fetal hepatic TG levels persisted at P180, concurrent with a 2-fold increase in percent body fat. Importantly, reversing the maternal HFD to a low-fat diet during a subsequent pregnancy improved fetal hepatic TG levels and partially normalized gluconeogenic enzyme expression, without changing maternal body weight. These results suggest that a developing fetus is highly vulnerable to excess lipids, independent of maternal diabetes and/or obesity, and that exposure to this may increase the risk of pediatric NAFLD.

Pioglitazone reduces ER stress in the liver: direct monitoring of in vivo ER stress using ER stress-activated indicator transgenic mice

It is known that endoplasmic reticulum (ER) stress is provoked under diabetic conditions and is possibly involved in the development of insulin resistance. In this study, using ER stress-activated indicator (ERAI) transgenic mice which express green fluorescent protein under ER stress conditions, we directly evaluated the effects of a diabetic agent pioglitazone on in vivo ER stress under diabetic conditions. In high fat and high sucrose diet-induced diabetic ERAI transgenic mice, 8 weeks of pioglitazone treatment reduced the accumulation of fat droplets in the liver and attenuated the development of insulin resistance. In the liver of the ERAI transgenic mice, ERAI fluorescence activity was clearly reduced as early as after 4 weeks of pioglitazone treatment, preceding the improvement of insulin resistance. In addition, after the pioglitazone treatment, serum free fatty acid and triglyceride levels were decreased, and serum adiponectin levels were increased. These data indicate that pioglitazone treatment suppresses ER stress in the liver which may explain, at least in part, the pharmacological effects of pioglitazone to reduce insulin resistance.

Insulin sensitivity: modulation by nutrients and inflammation

Insulin resistance is a major metabolic feature of obesity and is a key factor in the etiology of a number of diseases, including type 2 diabetes. In this review, we discuss potential mechanisms by which brief nutrient excess and obesity lead to insulin resistance and propose that these mechanisms of action are different but interrelated. We discuss how pathways that "sense" nutrients within skeletal muscle are readily able to regulate insulin action. We then discuss how obesity leads to insulin resistance via a complex interplay among systemic fatty acid excess, microhypoxia in adipose tissue, ER stress, and inflammation. In particular, we focus on the hypothesis that the macrophage is an important cell type in the propagation of inflammation and induction of insulin resistance in obesity. Overall, we provide our integrative perspective regarding how nutrients and obesity interact to regulate insulin sensitivity.

Endoplasmic reticulum stress and unfolded protein response in renal pathophysiology: Janus faces

A number of pathophysiological insults lead to accumulation of unfolded proteins in the endoplasmic reticulum (ER) and cause ER stress. In response to accumulation of unfolded/misfolded proteins, cells adapt themselves to the stress condition via the unfolded protein response (UPR). For the cells, UPR is a double-edged sword. It triggers both prosurvival and proapoptotic signals. ER stress and UPR may, therefore, be involved in a diverse range of pathological situations. However, currently, information is limited regarding roles of ER stress and UPR in the renal pathophysiology. This review describes current knowledge on the relationship between ER stress and diseases and summarizes evidence for the link between ER stress/UPR and renal diseases.

Effect of pioglitazone treatment on endoplasmic reticulum stress response in human adipose and in palmitate-induced stress in human liver and adipose cell lines

Obesity and elevated cytokine secretion result in a chronic inflammatory state and may cause the insulin resistance observed in type 2 diabetes. Recent studies suggest a key role for endoplasmic reticulum stress in hepatocytes and adipocytes from obese mice, resulting in reduced insulin sensitivity. To address the hypothesis that thiazolidinediones, which improve peripheral insulin sensitivity, act in part by reducing the endoplasmic reticulum stress response, we tested subcutaneous adipose tissue from 20 obese volunteers treated with pioglitazone for 10 wk. We also experimentally induced endoplasmic reticulum stress using palmitate, tunicamycin, and thapsigargin in the human HepG2 liver cell line with or without pioglitazone pretreatment. We quantified endoplasmic reticulum stress response by measuring both gene expression and phosphorylation. Pioglitazone significantly improved insulin sensitivity in human volunteers (P = 0.002) but did not alter markers of endoplasmic reticulum stress. Differences in pre- and posttreatment endoplasmic reticulum stress levels were not correlated with changes in insulin sensitivity or body mass index. In vitro, palmitate, thapsigargin, and tunicamycin but not oleate induced endoplasmic reticulum stress in HepG2 cells, including increased transcripts CHOP, ERN1, GADD34, and PERK, and increased XBP1 splicing along with phosphorylation of eukaryotic initiation factor eIF2alpha, JNK1, and c-jun. Although patterns of endoplasmic reticulum stress response differed among palmitate, tunicamycin, and thapsigargin, pioglitazone pretreatment had no significant effect on any measure of endoplasmic reticulum stress, regardless of the inducer. Together, our data suggest that improved insulin sensitivity with pioglitazone is not mediated by a reduction in endoplasmic reticulum stress.